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Polyphenols have been reported to modulate starch digestion, thus influencing the glycaemic index of high energy-density food. Modulation of starch digestion may be achieved through phenolic interactions with either digestive enzymes and/or the starch granules (Chapter 1). But the mechanisms can be much more complex when components other than starch are present in the food matrix. Therefore, it is essential to understand the complex mechanisms underpinning the role of polyphenols in limiting starch digestibility from different aspects. In this thesis, I studied the influence of the food matrix on the role of polyphenols in limiting starch digestibility (Chapter 2&3), mechanisms of starch-phenolics interactions (Chapter 4&5) and inhibition on brush border enzymes (Chapter 6).
Chapter 2 and 3 were designed to study the role of food matrix in limiting the inhibitory capacity of polyphenols on starch digestibility. In Chapter 2, co-digestion and fortification were used as strategies to investigate how the food matrix influences the role of berry polyphenols on starch digestibility. The inhibition obtained by co-digesting bread with berry extract is higher than the inhibition by digestion berry extract-fortified bread. Starch and gluten, as two main components in the bread matrix, interacted with berry phenolics to a large extent and that was indicated by the lower polyphenols bio-accessibility. On one hand, those interactions with food matrix (starch and gluten) lowered the amount of berry phenolics that can bind and inhibit α-amylase. On the other hand, starch-phenolics interactions themselves can contribute to slow down starch digestibility. This study shows co-digestion of phenolics with starchy food is a more efficient strategy to slow down starch digestibility in a bread matrix compared to bread fortification.
In Chapter 3, I further studied how the phenolic types and gluten presence affect the inhibitory capacity of polyphenols on starch digestibility. In this study, green tea and black tea were chosen as the polyphenols sources to study different types of polyphenols. Conventional wheat bread and gluten-free bread were prepared for studying the effects of gluten presence. Gluten is an important matrix components in wheat-based products. I found the presence of gluten lowered the inhibitory capacity of polymeric phenolics, but had little effect on the inhibitory capacity of monomeric phenolics. The results show that the tea polyphenols can be a promising strategy for modulating the glycaemic index of starchy food, but this strategy must be adapted to the selected food matrix.
In previous chapters, I have observed that interaction between starch and polyphenols may contribute to starch digestibility, so I studied further this interaction in Chapters 4&5. In Chapter 4, the interactions of wheat starch (WS) and tannic acid (TA) were investigated for their gelatinization, pasting, structural, and steady and dynamic rheological properties and digestibility of wheat starch. TA was either complexed with starch (WS-TA complexes) or mixed with starch (WS-TA mixtures) right before the characterization of its properties. A substantial amount of inclusion and non-inclusion complexes were produced and the production of both types of complexes is affected by the way of TA addition. Inclusion complexes were mostly produced by interaction of TA with non-gelatinized starch. Non-inclusion complexes were mostly produced by mixing TA with native starch followed by heating. The two different types of complexes influenced rheological properties of wheat starch differently, i.e., inclusion complexes produced weaker gel and non-inclusion complexes produced stronger gel. Both inclusion and non-inclusion complexes largely inhibited starch digestibility.
In Chapter 5, I studied the starch-phenolics interaction by using three types of starch, i.e., potato starch (B type), corn starch (A-type) and corn amylopectin (A-type). The starch-phenolics interactions influenced by starch types and amylose content were discussed. Similar to the addition method in Chapter 4, tannic acid was added by complexing and simply mixing with starch. The amount of TA in mixture samples is higher than that in complex samples. Besides, heat incubation (37ºC) during complex preparation influenced the functional properties of three starches and most significantly in potato starch, for instance, the XRD patterns changed from B-type to A- plus B-type. DSC peak at 120 ~160 ºC provided evidence of non-inclusion complexes formed by TA with all three types of starch. Gelatinized and non-gelatinized amylose and amylopectin showed different binding abilities to TA, i.e., non-gelatinized amylopectin has more ability to interact with TA than non-gelatinized amylose, but gelatinized amylose have more ability to interact with TA than gelatinized amylopectin. The more open structure of B-type starch makes it easier to interact with TA than tightly packed A-type starch, since physical-chemical properties of potato starch were more affected compared to corn starch and corn amylopectin. These results can help stimulate further interest in applications of starch-tannic acid interactions in various starchy food.
In Chapter 6, I studied the inhibitory effects of tea polyphenols on α-glucosidase. α-glucosidase is provided by rat intestinal extract and human Caco-2 cells. Various inhibitors were selected including epigallocatechin gallate, epicatechin gallate and four phenolics-rich tea extract from white, green, oolong, black tea. Four substrates were used to study inhibitory effects of tea polyphenols towards different hydrolytic activities including maltase, sucrase, isomaltase and glucoamylase. Maltase and glucoamylase from rats were largely inhibited by tea polyphenols, but rat isomaltase and sucrase were not. A mixed type of inhibition on rat maltase was found, which is composed of competitive and uncompetitive types of inhibition. Tea extracts in combination with acarbose, produced an antagonistic effect on rat maltase activity. Tea polyphenols show different sensitivity for rat and Caco-2 α-glucosidase. Epigallocatechin gallate and green tea extract exhibit dose-dependent inhibition on human sucrase, but no inhibition on rat sucrase. The opposite was observed when assessing maltase activity. The results highlighted the intrinsic difference among various enzymes and various testing conditions need to be taken into account when reporting inhibitory effects on α-glucosidase.
Finally, in Chapter 7, a summary of the main findings, some considerations of methodology and implications was provided. The most important conclusion is that the inhibitory effects of polyphenols on starch digestibility are influenced by multiple interactions among polyphenols, digestive enzymes, starch and other components in the food matrix.
|Qualification||Doctor of Philosophy|
|Award date||4 Oct 2021|
|Place of Publication||Wageningen|
|Publication status||Published - 2021|
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- 1 Examination/teaching third parties
Member of the Exam Committee of the PhD Thesis of Lijiao Kan
Walter Gerrits (Examiner)4 Oct 2021
Activity: Examination › Examination/teaching third parties › Academic
- 1 Finished
Anti-diabetes and anti-inflammation mechanism of physochemicals isolated form vegetables.
Kan, L., Fogliano, V., Capuano, E. & Oliviero, T.
1/09/17 → 4/10/21